% arara: lualatex
\documentclass{article}
\usepackage{luatexja-fontspec}
\setmainjfont{MS Mincho}
\newcommand{\HiraganaRo}{ろ}
\begin{document}
\[\HiraganaRo = 42\]
In the formula I use the symbol \HiraganaRo{} which is taken from the Japanese Hiragana alphabet.
\end{document}